Why do I see fractional spurs at multiples of 12.5Khz?

Document created by analog-archivist Employee on Feb 23, 2016Last modified by analog-archivist Employee on Aug 10, 2016
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I am using an ADF4350 in fractional mode (see attached *.pll file),but first
simulations gives
poor results for phase noise at 12,5kHz from the carrier and for fractional
spurious.
Could you help me to improve phase noise in my design ?

 

In your settings the Modulus=768 and it is divisible by 2,3 or 6. Therfore,
spurs will occur at one sixth of Fpdf/MOD, 19.2MHz/(768*6) = 4.1666Mhz,
8.333Khz, 12.5Khz etc.

Therefore, reduce Modulus to say 25 and the spurs will ocurr at greater offsets
from the carrier frequency, in this case 19.2Mhz/25 = 768Khz.

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